1. Field of the Invention
The invention relates to radiant energy, photocell arrays for capturing such energy, apparatus containing such photocells, including thermal infrared cameras, and more particularly to the optical or pre-photocell system, and to light valves, apertures, diaphragms, irises, and to the temperature control thereof. The method and apparatus of the invention shows particular utility in fire fighting, military, law enforcement, search and rescue, and agricultural applications or in any other application utilizing infrared imaging or detecting.
2. Background
The background of this invention involves thermal infrared cameras (throughout, the use of “infrared camera” is meant to be inclusive of “thermal infrared camera”), variable diaphragms and swappable fixed apertures. Infrared cameras are well known in the art. The typical infrared camera is comprised of at least several of the below described basic parts, with the most sophisticated infrared cameras comprised of most or all of these prior art parts. The variable diaphragm and swappable fixed aperture are also well known in the art, as are logic control systems.
Thermal infrared radiation is the emission of photons by all objects that are at a temperature above absolute zero. Thermal infrared radiation decreases very rapidly as the temperature of an object decreases.
Infrared Cameras:
Central to all infrared cameras is an infrared sensitive photocell. The photocell is highly sensitive to thermal infrared radiation (hereinafter referred to as “radiation”). The photocell is exposed to radiation emanating from the object or scene being imaged. However, the camera enclosure also emits radiation that can reach the photocell. This undesired radiation negatively affects the operation of the camera and therefore the photocell can be enclosed within a cold structure (referred to as a “radiation shield”). The design of the radiation shield is dictated simply: if an observer were to look out from the photocell, anything the observer could see would emit radiation that would arrive at the photocell. In order for the radiation shield to block the undesired radiation, it must be the only internal camera structure that the photocell can “see.” The “cold stop,” which is simply a name for a cooled aperture, provides the only path for external radiation to reach the photocell, through the focusing optics. The radiation shield should therefore also not emit an excess of radiation itself. The cold stop size is a compromise between the effectiveness of blocking the unwanted radiation (requiring a small aperture) and excessive vignetting (requiring a large aperture).
In order to keep the photocell and other components of the infrared camera cool, there is often an active cooler integrated into the camera. Typically, the cooling system must maintain a fixed temperature in order to control the unwanted radiation seen on the photocell, although ideally the radiation shield is cold enough to produce a negligible amount of radiation onto the photocell. This fixed temperature has a known effect on the photocell that can be removed through image post processing. The photocell is also cooled to improve its radiation sensitivity and reduce the internally generated current, as the higher the temperature of the photocell, the lower its usable dynamic range. A thermoelectric cooler, such as a Peltier cooler, typically cools such infrared cameras. However, in more sensitive systems where the signal to noise ratio is far more important, the system must be cooled to as low a temperature as reasonably possible, to minimize any unwanted radiation loading. In these systems, there are several options for achieving the necessary cooling, including integrating the cameras into dewars for liquid nitrogen or liquid helium, Stirling cryogenerators, Gifford-McMahon mechanical coolers, and other such devices.
To reduce thermal load on the cooling system, infrared camera designers often place all of the cooled elements into a vacuum vessel. Within the vacuum vessel, the radiation shield and the photocell are maintained at a low, sometimes cryogenic, temperature, based on the photocell requirements and the desired performance. The vacuum vessel, (if one is present) often constitutes a camera housing, which also often contains, or provides, a mounting apparatus for the infrared focusing lens (throughout, “lens” is inclusive of all light collecting devices including refractive or reflective systems).
Thermal infrared cameras must be able to accommodate both hot and cold target objects and scenes, while distinguishing target from background radiation. Although the thermal control methods described above can allow a camera to be used in a wide variety of thermal scenes, drastic changes in radiation quantities require different camera settings. If the scene is too cool for ideal use with the camera, the camera operator can take a longer exposure. This method may adversely affect the frame rate and may lead to resolution problems if the camera or target is moving. Another solution typically used in the art is to change the electronic gain of the signal from the photocell, although a higher gain also increases the noise in the electronic signal. Conversely, in hot scenes, reduced exposure time, reduced signal gain, or a combination of the two can allow an infrared camera to capture the scene.
Apertures and Cold Stops:
A cold stop is simply a temperature-controlled aperture. In its most basic form, the cold stop is a fixed aperture, similar to the aperture found in some disposable light cameras. Variable diaphragms (hereinafter used interchangeably with an “iris”) and swappable fixed apertures for light cameras have been described in patent art for many years (see e.g., U.S. Pat. No. 24,356 to Miller and Wirsching in 1859, U.S. Pat. No. 582,219 to Mosher in 1897). The variable diaphragm works by allowing more or less of the radiation (visible light, in the case of visible light cameras) that reaches the focusing lenses to pass through to the photocell or film. The focusing lens receives radiation and focuses it based on the distance from the radiation source to the lens and the prescription of the lens. The prescription includes the focal length and the f-number. In conventional light cameras, the aperture is typically built into the compound lens assembly. That aperture then lets pass a certain desired portion of the radiation intercepted by the lens.
With a very large aperture, nearly all of the light arriving at the focusing lens passes through the aperture. By reducing the size of the aperture, the mechanism of the aperture itself blocks a portion of the light from entering. In typical light cameras, the aperture is located at the point where the cone of light from the object is wide and thus diminishes the light intensity without affecting the image quality. Lenses may have specific aperture requirements, which determine the optimum position and size of the aperture. This is typically a function of the f-number (hereinafter interchangeably also referred to as “f/#”), the focal length of the lens, and the construction. However, in infrared cameras, the aperture cannot be located in the lens since the lens is uncooled. The aperture is typically located in the converging path of the light; that is, between the lens and the focal plane, so the aperture first reduces the image intensity and then, with reducing aperture size, begins to vignette, or cut off, the outer edges of the image. The aperture thus defines an effective f/# for the system.
As a result, when interchangeable lenses of a different f/# are used with an infrared camera, the system f/# may not match the lens f/#. There is heretofore no solution to this problem in the prior art. A variable diaphragm or aperture, however, can correct this situation and match the system f/# to the specific lens in use. By lens, we refer to all light collecting devices including refractive or reflective systems.
U.S. Pat. No. 6,133,569 to Shoda and Ishizuya discloses a thermal infrared camera incorporating all of the above-mentioned features. The Â□569 patent further describes the promising idea of using variable diaphragms in thermal feedback infrared cameras, that is, in cameras with thermal sensors controlling cooling elements. Specifically, Shoda and Ishizuya suggest the use of an optically variable diaphragm optionally thermally coupled to the infrared radiation shield. However, due to the limitations discussed below in regards to cooling the variable diaphragm, the Â□569 patent has not made possible the use of such a variable diaphragm.
The use of continuously variable diaphragms or swappable fixed apertures in thermal infrared cameras has to date not been viable because of fundamental packaging and thermal control problems. As described above, the aperture must be cooled. While an effectively cooled variable diaphragm is difficult to design, the problem becomes considerably more difficult if the aperture must be kept at cryogenic temperatures and located inside a vacuum chamber. Within a vacuum chamber, the aperture and the associated drive mechanisms cannot outgas. Depending on the depth of vacuum, this may require a completely dry iris and specially designed lubricants, electrical wiring, motors, and gears. Moreover, the drive mechanism cannot add heat load onto the cooling system, nor allow conductive heat load from the ambient vacuum enclosure to affect the cooling system. Equally important, the aperture must dissipate energy from the radiation that it blocks. These and other considerations for the aperture itself have made implementing a variable diaphragm impossible given the prior art.
Further, with continuously variable diaphragms or swappable fixed apertures, there must be some mechanism for changing the aperture. There must be mechanical, electromagnetic, piezoelectric, or other such control means to change the diaphragm size or swap fixed apertures. The control means must be strong enough to operate the variable diaphragm or swappable fixed aperture in a timely manner, and either thermally isolated from the photocell or able to operate at cryogenic temperatures. As mentioned above, if the aperture is in a vacuum, the control means must be small enough to contain within the vacuum chamber or must provide a means for transferring mechanical force through the wall of the vacuum chamber. Where such transfer of mechanical force occurs, there must be complex seals to ensure the vacuum is not compromised and that excessive heat is not conducted into the radiation shield.
Aperture control means located in a vacuum chamber have several constraints that make their implementation significantly less feasible. First, the materials used in conjunction with the control means cannot outgas, as vaporized materials not only destroy the vacuum that provides the thermal isolation for the cold components, but also condense on the photocells. For this reason, bearings, linings, coatings, winding insulation, and any cements or glues must be eliminated or replaced with a fluorinated polymer or polytetrafluoroethylene based insulation or otherwise coated or manufactured with special non-outgassing materials.
Moreover, the motor control means must also be able to cool itself effectively without the typical convection of heat into air. This means that all heat generated in the motor must be dissipated through conduction to the motor mounting apparatus. The control means must therefore be thermally isolated from the aperture it controls. The motor must incorporate heat-reducing technology, including bipolar drives, low current standby systems, and other such options. Furthermore, the diaphragm control means must not produce electromagnetic interference (EMI) that can distort the electronic signal produced by the photocell. Mechanical or other temperature control means must often also be associated with the motor.
Finally, for control means located in a vacuum, there is an additional potential problem created by high voltage to exposed conductors in the motor apparatus. In extremely low-pressure vacuums, the remaining air molecules subject to high voltage can ionize and current will flow as if the vacuum chamber were an electron tube, creating strong corona effects. These effects are particularly problematic near highly sensitive photocells, so careful insulation is needed on any exposed electric contacts.
An additional packaging problem exists where a variable diaphragm system must fit within the same confines as an existing fixed aperture camera. In these retrofit cases, the entire aperture control means must fit within very small confines that were not designed to accommodate such hardware.
Accordingly, there is a need in the art for a continuously variable diaphragm or swappable fixed aperture along with a detailed method of implementing such a means that overcomes or avoids the above problems and limitations.